Currently, problems such as the depletion of fossil fuel, environmental pollution, and global warming according to the high-speed growth have been increasing. As a countermeasure thereto, a new and renewable energy has been developed, but notable outcomes have not been achieved. Accordingly, an interest in an energy storage technology, particularly a battery field, has increased.
For instance, advanced development has been achieved in a lithium ion battery. However, the conventional lithium ion batteries are insufficient to replace fossil fuel due to a low energy density thereof. In addition, recently, a metal-air battery such as a lithium-air battery has been actively developed.
The lithium-air battery uses oxygen supplied from air without any restriction as an active material. Theoretically, sufficiently high energy density may be obtained from the lithium-air battery, for example, of about 3,200 Wh/kg, which is about 10 times greater than that of the conventional lithium ion battery. Further, since oxygen is used as the active material, the lithium-air battery is environmentally friendly.
However, the conventional lithium-air battery has a drawback of a short life-span and an over-voltage due to high polarization. In particular, when the battery is discharged, lithium peroxide (Li2O2) may be generated and ion conductivity of lithium peroxide is low. Therefore, when lithium peroxide covers an air electrode (cathode) of the lithium-air battery, high polarization occurs and thus energy efficiency is remarkably reduced. Accordingly, the an important research subject in the lithium-air battery may be the development of an appropriate catalyst decomposing lithium peroxide.
An early lithium-air battery of the related art used a solid oxygen generation catalyst. Since the aforementioned catalyst was a solid phase, it was difficult to bring the catalyst into contact with lithium peroxide which was the same solid phase. As a result, since activity of the catalyst was low, the aforementioned problem could not be solved. Recently, research for a soluble catalyst (soluble catalyst) has been actively conducted. Since the soluble catalyst may freely move in the electrolyte, the soluble catalyst may come into contact with lithium peroxide more easily.
The soluble catalyst includes a redox mediator decomposing lithium peroxide while being oxidized and reduced. For instance, it has been reported in the related art that tetrathiafulvalene (TTF) as the redox mediator of the soluble catalyst for the lithium-air battery. Further, the present inventor has suggested lithium iodide (LiI) as another alternative that may be usable as the redox mediator. Accordingly, the present inventor has strived to concentrate on research for developing a novel advanced soluble catalyst solving the problems of the lithium-air battery, thereby reaching the present invention.
The above information disclosed in this Background section is merely for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.